tests/azure/arm: Update test image name (#10133)
tests/azure/arm: Update test image name (#10133)

I've run into a few cases where running the full test suite for the ARM
builder would fail because of conflicting image names. This is a
workaround for now the bigger fix is to have the acceptance clean up any
created images.

Results before change
```
the managed image named testBuilderAccManagedDiskWindows-1603151855 already exists in the resource group packer-acceptance-test, use the -force option to automatically delete it.
--- FAIL: TestBuilderAcc_ManagedDisk_Windows_Build_Resource_Group (0.66s)
```

Results after change
```
PASS
ok      github.com/hashicorp/packer/builder/azure/arm   2431.115s

```
ricardclau/packer,mitchellh/packer,ricardclau/packer,rnaveiras/packer,ricardclau/packer,rnaveiras/packer,hashicorp/packer,rnaveiras/packer,pecigonzalo/packer,mitchellh/packer,ricardclau/packer,hashicorp/packer,mitchellh/packer,hashicorp/packer,rnaveiras/packer,mitchellh/packer,mitchellh/packer,rnaveiras/packer,ricardclau/packer,pecigonzalo/packer,ricardclau/packer,hashicorp/packer,pecigonzalo/packer,pecigonzalo/packer,rnaveiras/packer